Occupant detection and classification systems use various metrics to detect and classify vehicle occupants. The metrics can be measured or based on a user input. Sometimes, the metrics relate to the occupant's size. For instance, an occupant detection system may distinguish adult occupant size based on a seatbelt payout. Present occupant classification systems may determine seatbelt webbing payout length based on indirect measurements, e.g., spindle diameter, webbing thickness, webbing on spindle, film spool effect, etc. Present mechanisms for measuring seatbelt payout can be difficult, inefficient, and/or costly to implement.